1. Field of the Invention
The present invention relates to an image forming device. More particularly, the present invention relates to a transfer sheet conveyance and guide mechanism of an image forming device, and specifically to a transfer sheet conveyance and guide mechanism which guides a transfer sheet fed upward from a pair of paper stop rollers to a transfer nip between a photoconductive drum and a transfer roller.
2. Background Information
In an image forming device utilizing an electrophotographic system, a charging unit, an exposure unit, a developing unit, a transfer unit, a cleaning unit, and the like are arranged around a photoconductive drum as an image carrying member. A fixing device is provided downstream of the photoconductive drum in a transfer sheet conveying direction. In such an image forming device, the charging unit will first uniformly charge the surface of the photoconductive drum. Next, the exposure unit exposes the photoconductive drum based on image data to form an electrostatic latent image on the surface of the photoconductive drum. The electrostatic latent image is developed by the developing unit into a toner image, which is transferred to a transfer sheet by the transfer unit. The toner image on the transfer sheet is fixed by means of a fixing unit, and the transfer sheet on which the image is formed is discharged to a discharge unit.
One type of transfer unit in an image forming device is a transfer roller that is disposed so as to be in contact with the photoconductive drum and form a transfer nip therebetween. A transfer sheet is transported from a feeding unit to the transfer nip through a pair of paper stop rollers. At the transfer nip, a transfer bias voltage is applied to the transfer roller to transfer the toner image carried on the photoconductive drum to the transfer sheet. In order to obtain a fine image by precisely transferring the toner image held on the surface of the photoconductive drum to the transfer sheet, it will be necessary to precisely guide the transfer sheet transported from the pair of paper stop rollers to the transfer nip. Thus, a pair of pre-transfer guide plates are provided upstream of the transfer nip in a transfer sheet conveying direction in order to precisely guiding the transfer sheet transported from the pair of paper stop rollers to the transfer nip. Here, one of the pair of the pre-transfer guide plates is disposed on the transfer roller side, and the other is disposed on the photoconductive drum side, and the pre-transfer guide plate provided on the photoconductive drum side is connected to ground.
Recently, as multifunction devices having the functions of a copying machine and a printer have increased in number, there are more and more devices having a vertical transport path for upwardly transporting a transfer sheet, in which an image forming unit, a fixing unit, and so on are disposed in a vertical direction, in response to the need to miniaturize the device and speed up image formation. In a miniaturized image forming device having such a vertical transport path, a photoconductive drum and a transfer roller are designed to have smaller diameters. Accordingly, the space between the transfer roller and the pre-transfer guide plate provided on the transfer roller side, and the space between the pre-transfer guide plate provided on the transfer roller side and the pre-transfer guide plate provided on the photoconductive drum side are narrower.
Here, if an electrically conductive member is used as the material that forms the pre-transfer guide plate, when a transfer bias voltage is applied to the transfer roller in an image forming process, transfer bias current flows through the pre-transfer guide plate provided on the transfer roller side to ground. If the transfer bias current thus flows to ground, insufficient transfer potential will be applied from the transfer roller to the transfer sheet, thus generating a defective transfer. In addition, if an electrical insulator is used as the material that forms the pre-transfer guide plate in order to prevent the transfer bias current from escaping, when the transfer sheet passes by the pre-transfer guide plate, charge storage due to triboelectrification between the transfer sheet and the guide plate will occur, thus generating gray streaks on a formed image.
In the device shown in Japanese Patent Application Publication No. 11-219042, the transfer bias current is controlled in order to prevent defective transfer and transfer void.
In this conventional device, after a transfer sheet is fed into the transfer nip, current leakage from the transfer roller to the pre-transfer guide plate via the transfer sheet will be detected, and a controller will change the transfer bias to be applied to the transfer roller based on the detection result. However, in this device, both a detection circuit for detecting the current flowing into the pre-transfer guide plate and a controller have to be provided, resulting in an increased cost of the overall device. Additionally, if the pre-transfer guide plates are chipped due to friction with a transfer sheet in the image forming process, or if paper dust from the transfer sheets is adhered to the pre-transfer guide plates, the amount of current detected by the detection circuit will varies from an initial value, making it difficult to control the current.
In view of the above, there exists a need for a transfer sheet conveyance and guide mechanism of an image forming device which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.